As the need for distributed information becomes greater and greater, there is likewise a need for distributed, high power transmission line systems to handle this increasing communication traffic. A typical distributed transmission line system comprises a main transmission line and one or more tap lines coupled to the main transmission line for receiving signals from the main transmission line. A familiar distributed transmission line system is the coaxial cable system which brings cable TV to television sets. More sophisticated distribution systems are bi-directional, transmitting and receiving signals of many services located at various frequencies throughout the radio frequency spectrum. These signals include FM radio, PCS and cellular phone services. A typical distributed communications application employs radiating coaxial cables in underground subway systems. To add branch lines to the main coaxial cable, non-directional taps or couplers are required. The operational frequency range of the taps must span the range of these various services, which are located between 88 and 2000 MHz.
Some of the difficulties associated with providing non-directional tap couplers over a broad frequency bandwidth are as follows. First, it is difficult to provide a non-directional tap coupler which can maintain a relatively constant coupling ratio (power transmitted by tap line/power transmitted by main transmission line) over the broad frequency spectrum. Second, it is difficult to provide a non-directional tap coupler in which the voltage standing wave ratio (VSWR) is kept close to the minimum value of 1. VSWR is the ratio of the maximum voltage to the minimum voltage on a line. As known, when a load (R.sub.L) matches the characteristic impedance (Z.sub.0) of a line, there is no reflected wave so that V.sub.max equals V.sub.min and the VSWR is 1. It is important to minimize the VSWR to keep losses low.
Finally, even if the difficulties associated with providing non-directional tap couplers over a broad frequency bandwidth discussed above are overcome, it must be done in a manner which provides a tap line within a "practical characteristic impedance range". As known, the greater the characteristic impedance of a line, the narrower the width of the line, and vice versa. Thus, if the characteristic impedance of the tap line is a high value, this will require an ultra thin (describable in terms of hair-width or thinner) tap line; thereby making the manufacture of the non-directional tap coupler very difficult and expensive. Furthermore, an ultra thin tap line is limited in terms of its power handling capability. Therefore, the characteristic impedance of the tap line must fall within the "practical characteristic impedance range," which is defined herein as the range of characteristic impedances which provide tap lines with sufficient power handling capability to transmit signals throughout the radio frequency spectrum and which do not require an ultra thin tap line that is practically unfeasible to manufacture due to the difficulty and expense involved.
What is desired, therefore, is a non-directional tap coupler for use in high power operation over a wide bandwidth which minimizes losses while providing a tap line within a practical characteristic impedance range.